Implemented DeleteSpatialCommand.
Merge branch 'master' of github.com:/structr/structr

Conflicts:
	structr-rest/src/main/java/org/structr/rest/maintenance/DeleteSpatialIndexCommand.java
Added maintenance command to fix corrupted spatial indexes, added some null checks in EntityContext.
Merge branch 'develop'
Merge branch 'develop'
Removed duplicate ID serialization from StreamingJsonWriter.
Merge branch 'develop'
Merge branch 'develop'
Merge branch 'develop'

Conflicts:
	structr-core/src/main/java/org/structr/core/EntityContext.java
added owner property to ui view
some progress on partial rendering, but creating partials
for markup elements is disabled for now, needs rework
some minor bug fixes
Merge branch 'develop'
disabled login with empty string as password
set session id in case of successful registration
new text function 'abbr(str,maxlen)' to shorten text to maxlen,
does not cut words
abbreviate text only if longer than maxlen